Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 30 July 2019 doi:10.20944/preprints201902.0215.v2 1 Testing host-plant driven speciation in phytophagous insects: a phylogenetic perspective 2 3 Emmanuelle Jousselin* 1 , Marianne Elias 2 4 1. CBGP, INRA, CIRAD, IRD, Montpellier SupAgro, Univ Montpellier, Montpellier, France 5 2 Institut Systématique Evolution Biodiversité (ISYEB), Muséum national d'Histoire naturelle, 6 CNRS, Sorbonne Université, EPHE, Université des Antilles, 57 rue Cuvier, CP 50, 75005 Paris, 7 France. 8 *corresponding author: [email protected] 9 10 Keyword: coevolution, herbivory; host-plant specialization, phylogeny, speciation. 11 12 Abstract 13 During the last two decades, ecological speciation has been a major research theme in 14 evolutionary biology. Ecological speciation occurs when reproductive isolation between 15 populations evolves as a result of niche differentiation. Phytophagous insects represent model 16 systems for the study of this evolutionary process. The host-plants on which these insects feed and 17 often spend parts of their life cycle constitute ideal agents of divergent selection for these 18 organisms. Adaptation to feeding on different host-plant species can potentially lead to ecological 19 specialization of populations and subsequent speciation. This process is thought to have given 20 birth to the astonishing diversity of phytophagous insects and is often put forward in 21 macroevolutionary scenarios of insect diversification. Consequently, numerous phylogenetic 22 studies on phytophagous insects have aimed at testing whether speciation driven by host-plant 23 adaptation is the main pathway for the diversification of the groups under investigation. The 24 increasing availability of comprehensive and well-resolved phylogenies and the recent 25 developments in phylogenetic comparative methods are offering an unprecedented opportunity to 26 test hypotheses on insect diversification at a macroevolutionary scale, in a robust phylogenetic 27 framework. Our purpose here is to review the contribution of phylogenetic analyses to investigate 28 the importance of plant-mediated speciation in the diversification of phytophagous insects and to 29 present suggestions for future developments in this field. 1 © 2019 by the author(s). Distributed under a Creative Commons CC BY license. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 30 July 2019 doi:10.20944/preprints201902.0215.v2 30 Introduction 31 The idea according to which new species arise through adaptation to different ecological 32 niches constitutes the core of Darwin’s work. This process is now termed ecological speciation 33 and its study has become an intense field of research in evolutionary biology (Nosil, Crespi, 34 Sandoval 2002; Rundle, Nosil 2005; Schluter 2009; Nosil 2012). Phytophagous insects*1 have 35 always been at the forefront of these investigations (Drès, Mallet 2002; Funk, Filchak, Feder 2002; 36 Matsubayashi, Ohshima, Nosil 2010; Elias et al. 2012; Forbes et al. 2017). The hypothesis of 37 ecological speciation resulting from divergent selection exerted by host-plants was put forward a 38 long time ago to explain the formation of new species of insects (Brues 1924). There are several 39 model systems on which this scenario has been explored. One text-book example of host-plant 40 driven incipient speciation is the apple maggot (Rhagoletis pomonella complex) in which the 41 evolution of new feeding preferences on the recently introduced domesticated apple (Malus 42 pumila) has supposedly led to the emergence of specialized host races (Bush 1975; Berlocher 43 2000; Powell et al. 2014). Stick insects, leaf beetles (Nosil et al. 2012; Rundle et al. 2000), 44 butterflies (McBride, Singer 2010), and the pea aphid also star among model systems in the study 45 of host-driven speciation (Via, Bouck, Skillman 2000; Caillaud, Via 2000; Peccoud et al. 2009; 46 Smadja et al. 2012). In all these examples, the speciation scenario hypothesizes that: 1) the 47 restricted utilization of distinct sets of host-plant species by insect populations is the result of 48 adaptive trade-offs; 2) hybrids with intermediate phenotypes (in terms of traits involved in host- 49 plant adaptation*) fare poorly on parental host-plants and are selected against and therefore gene 50 flow between populations is reduced; 3) gene flow can further be reduced through the evolution of 51 assortative mating, especially when host-plants also represent mating sites. In support of this 52 scenario, many studies show the existence of genetically differentiated host races in insect species. 53 Some studies have quantified selection against hybrids (McBride, Singer 2010; Gow, Peichel, 54 Taylor 2007) and some studies have uncovered genomic regions that determine host-plant 55 preference and performance on alternative hosts (Egan, Nosil, Funk 2008; Smadja et al. 2012). 56 The role of host-plant-mediated speciation in the diversification of phytophagous insect 57 lineages is also largely emphasized in the literature on large-scale patterns of insect diversity: 58 macroevolutionary perspectives on phytophagous insect evolution have attributed their 59 extraordinary diversification to selective responses to their host-plants (Ehrlich, Raven 1964; 60 Yokoyama 1995; Winkler, Mitter 2008; Janz 2011). However, these macroevolutionary scenarios 61 are often presented in the literature as narratives for specific lineages commenting a phylogenetic 1 See glossary 2 Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 30 July 2019 doi:10.20944/preprints201902.0215.v2 62 reconstruction of the history of host-plant associations. Many phylogenetic studies still fail to 63 clearly formulate hypotheses and predictions about the speciation processes that underlie the 64 observed patterns and the role played by host-plant adaptation in those. The reason might be that 65 the macroevolutionary patterns that arise when host-plant specialization* is the driver of 66 speciation events are not always clear. There is no review on what to expect and how to formally 67 test these predictions. 68 The increasing availability of robust molecular phylogenies and recent developments in 69 phylogenetic comparative methods are offering an unprecedented opportunity to test evolutionary 70 hypotheses in a robust phylogenetic framework. Our purpose here is to present the 71 macroevolutionary scenarios for the diversification of phytophagous insects that have been put 72 forward in the literature, decipher the role that ecological speciation driven by host-plant 73 adaptation play in them and synthetize predictions from these scenarios. We then identify tools 74 from the “comparative phylogenetic toolbox” that provide ways to test some of these predictions. 75 This toolbox can be divided into three compartments: 76 1) comparisons of the phylogenetic histories of insects and their associated plants: the 77 congruence (in terms of dates of divergence and branching patterns) of the phylogenetic histories 78 of plant-feeding insects and their host-plants can be tested in robust statistical frameworks and 79 illuminate how herbivores track the diversification of their hosts; 80 2) ancestral character state reconstructions: the evolutionary trajectory of host-associations, 81 host breadth and host-plant adapted traits can be inferred using ancestral character state 82 reconstruction methods and statistical tests can determine whether their distribution throughout the 83 phylogenetic trees follow the predictions of scenarios involving ecological speciation mediated by 84 host-plant adaptation; 85 3) diversification analyses: the recent developments of methods to study the diversification 86 dynamics* of entire clades using phylogenetic trees provide ways to test how shifts to new host- 87 plant species or changes in host breadth have impacted diversification rates in phytophagous 88 insects. 89 We review papers that have adopted these approaches. We then present suggestions for future 90 research that should help linking microevolutionary studies on host-plant adaptation and 91 macroevolutionary perspectives on phytophagous insect diversification. 92 I Macroevolutionary scenarios of phytophagous insect diversification 93 I.1) Escape and radiate (Figure 1 a) 3 Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 30 July 2019 doi:10.20944/preprints201902.0215.v2 94 More than 50 years ago, Ehrlich & Raven (Ehrlich, Raven 1964) put forward a 95 macroevolutionary scenario that inspired most of the current research on plant-feeding insect 96 diversification: it is known as “Escape and radiate” (Thompson 1989). They hypothesized that 97 when insects acquire the ability to circumvent the chemical defenses of a plant group, it promotes 98 their rapid diversification by ecological release, i.e. the availability of novel resources and 99 reduction in direct competition. Insects undergo an adaptive radiation*. In this scenario, 100 adaptation towards host-plants is the driving force of insect species formation. The “Escape and 101 radiate” scenario also hypothesizes that, in response to phytophagous insect predation, plants 102 acquire novel chemical defenses which allow them in turn to diversify very rapidly (Marquis et al. 103 2016). Ehrlich & Raven’ s seminal study suffers from several shortcomings that have been 104 pinpointed before (Janz 2011). First, although the authors frame their theory within the concept of 105 adaptive radiation, they do not explicitly lay